1. Field of the Invention
The invention relates to a method of stabilizing natural composites or wood polymer composites (WPC) against damage by heat and light by treating the wood derived fillers or flour with a water based solution, suspension dispersion or emulsion of a sterically hindered amine, drying the treated wood derived filler and then combining with plastic via compounding methods such as extrusion.
2. Description of Related Art
Natural product composites, for example wood/plastic composites, are well known in the art. They may for example be natural fiber/plastic composites as disclosed in U.S. Pat. Nos. 6,290,885 and 6,511,757 hereby incorporated by reference.
Wood is a complex polymeric material containing essentially cellulose, hemicellulose and lignin. Lignin itself is a complex mixture of high molecular weight products which are derived from conyferyl alcohol.
Particularly lignin causes discoloration and undergoes degradation upon exposure to actinic radiation.
Polymeric materials are well known to degrade upon the action of sunlight. Hindered amines are well known stabilizers for polymers and sunlight protectants. See “Plastics Additives Handbook” 4th edition, R. Gachter et al. ed., Hanser Publishers, New York, pp. 129-263, (1993).
It is known to stabilize wood surfaces by applying hindered amines or nitroxyls to a wood substrate. For example EP-A-943 665 discloses a wide range of sterically hindered amine N-oxyls or N-hydroxyls which are applied to the wood substrate as solutions, suspensions or dispersions comprising at least one organic solvent.
U.S. Pat. Nos. 6,187,387 and 6,653,324 disclose the use of sterically hindered amine N-oxyls or N-hydroxyls or salts thereof which are applied directly or via impregnation to a wood substrate and are herein incorporated by reference.
U.S. Pat. Nos. 6,254,724 and 6,650,303 disclose the use of nitroxides and hydroxylamines in the stabilization of paper or pulp.
U.S. published applications 2002056534 and 2003121630 discuss a method for the production of colored light-stable, lignocellulosic material.
Attempts have been made to stabilize natural product composites by using hindered amine light stabilizers. See for example, Falk, R. H. et al., Natural Polymers and Composites, Proc. 3rd Int. Symposium on Natural Polymers and Composites, Sao Pedro, Brazil, May 14-17, (2000), 382-385.
Star, Nicole M. et al, Annual Technical Conference, Soc. Plast. Eng, 60th/Vol. 2, (2002), 2209-2213 shows compositions of wood flour and high density polyethylene in combination with hindered amines.
U.S. application publication number US 20040235983 discloses natural product composites protected against weathering and color formation by the incorporation of sterically hindered amines into the polymer-wood matrix.
U.S. application publication number US 20040076847 discloses pretreated wood filler with pigment and/or dyes before incorporation into a composite.
It has now been found that selected sterically hindered amines have an improved stabilizing activity against light-induced degradation of wood-polymer composites if they are applied directly to the cellulosic or wood component of the composite prior to combining the cellulosic or wood component with the plastic or polymeric component of the composites. The application of the solution, suspension or dispersion of hindered amines onto the cellulosic material occurs in the absence of matrix polymer. The treated cellulosic material is then combined with the polymer to form the composite.
While not wishing to be bound by any theory, it is known that hindered amines function as good radical scavengers. It is further believed that the prior impregnation of the cellulosic or wood component with the hindered amine places the radical scavenger at the lignin sites and thus provides the most effective protection for the lignin from discolorating light damage.
The present invention provides superior weatherability and protection from light induced discoloration to composite substrates as compared to prior art techniques.